In recent years, the development of semiconductor devices which use an oxide semiconductor for a channel in place of amorphous silicon, low temperature polysilicon and single crystal silicon has been proceeding (for example, Japanese Laid Open Patent Publication No. 2013-012603). A semiconductor device which uses an oxide semiconductor as a channel can be formed with a simple structure and at a low temperature process similar to a semiconductor device which uses amorphous silicon as a channel. It is known that a semiconductor device which uses an oxide semiconductor as a channel has higher mobility than a semiconductor device which uses amorphous silicon as a channel. It is known that a semiconductor device which uses an oxide semiconductor as a channel has a low off-current.
For example, an oxide semiconductor represented by an oxide semiconductor (In—Ga—Zn—O) including indium (In), gallium (Ga), zinc (Zn) and oxygen (O) has a band gap of 3.0 eV or more. Furthermore, the oxide semiconductor is transparent in the visible light region. Therefore, when a semiconductor device in which an oxide semiconductor is used for a channel is applied to a display device, the display device is hardly affected by light such as a backlight, and hardly causes malfunctions.
In a general display device, a transistor structure of a pixel circuit for displaying an image is a structure which is easily affected by light. On the other hand, a transistor structure of a drive circuit for driving a pixel circuit is a structure which is hardly affected by light. Therefore, in the case when a transistor including an oxide semiconductor is used for a display device, it is preferred to use an oxide semiconductor which has a large band gap and is hardly affected by light as a transistor of a pixel circuit. On the other hand, it is preferred to use an oxide semiconductor which has high mobility as a transistor of the drive circuit. In order to satisfy these conditions, oxide semiconductors which have different materials are used for each of a transistor of a pixel circuit and a transistor of a driver circuit. However, oxide semiconductors have a low chemical resistance to acid compared to silicon. As a result, it is difficult to manufacture a plurality of transistors in which oxide semiconductors having different materials are used for a channel.